Thin film filters having a negative temperature drift coefficient and method making the same

ABSTRACT

Methods for making thin film filters having a negative temperature drift coefficient are the subject of the present invention. Such filters can achieve better optical control within an operational temperature range from −5 to 70 degrees centigrade. A first embodiment of the present invention includes the steps of: 1. providing a substrate wafer which has a coefficient of thermal expansion (CTE) greater than that of a selected film stack material; 2. polishing the substrate wafer; 3. depositing thin film layers made of the film stack material on the substrate wafer at a temperature substantially higher than room temperature; 4. cooling the substrate-film stack laminate to room temperature, thus forming a convex-shaped laminate; 5. cutting the cooled laminate into pieces. A second embodiment includes the steps of: 1. providing a laminate compose of a glass substrate and a film stack; 2. using at least one ion beam source to bombard the film stack of the laminate with high energy ions; 3. cutting the bombarded laminate into pieces.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to thin film filters and the methodmaking the same, and particularly to thin film filters having a negativetemperature drift coefficient which can achieve better control of theoptical performance of a DWDM system when working within the operationaltemperature range. The related invention record was filed in PTO withdisclosure document no. 495113 on Jun. 12, 2001.

[0003] 2. Description of Related Art

[0004] In recent years, thin film filters have often been used inoptical systems for signal processing or optical communications. Thefilters operate to select light of desired wavelengths, often within anarrow band. Thin film filters may be used in association with gradientrefractive index (GRIN) lenses and optical fibers to form a densewavelength division multiplexing (DWDM) device. Referring to FIG. 5, theoperating principle of an eight-channel, filter type DWDM device isillustrated. Ideally, a light beam of a particular wavelength isconsidered one channel. In practice, on e channel is defined by a verynarrow range of wavelengths. The more channels a DWDM device has, thenarrower the pass bandwidth of each channel is.

[0005] To obtain narrower pass bandwidths, more layers of film arenormally deposited on a glass substrate, creating a stack of films onthe substrate. However, this procedure inevitably induces more internalstress in the film stack. The more tensile stress endured by a filmstack, the looser the atomic structure of the films in the stack.Interfaces between film layers in the stack act as mirrors, which act toseparate the wavelengths of a light beam. A looser atomic structure in afilm stack lowers the reflectivity of these interfaces. Thus tensilestress in a film stack acts to broaden the pass bandwidth. Conversely,the more compressive stress endured by a film stack, the narrower thepass bandwidth of the filter is.

[0006] The coating process is designed to minimize pass bandwidth driftat room temperature (23° C.). T he operational temperature range of athin film filter is from −5° C. to 70° C. Within this temperature range,the stress in the filter varies substantially linearly with thetemperature. FIG. 2 shows pass bandwidth of a filter at roomtemperature. Alcatel's 1915 LMI 10 mw WDM thin film filter has apositive temperature drift coefficient, 1 pm/° C. FIG. 3 shows how thepass bandwidth of Alcatel's 1915 LMI changes with a change intemperature. When the 1915 LMI's temperature increases from 23° C. to70° C., a 47 pm pass bandwidth enlargement occurs, and when thetemperature decreases from 23° C. to −5° C., a 28 pm pass bandwidthreduction occurs as is illustrated in FIG. 3. Obviously, sincetemperature fluctuation and resulting pass bandwidth drift areinevitable, it is preferable if pass bandwidth is reduced more oftenthan it is increased as the environmental temperature changes.Consequently, referring to FIG. 4, there is a demand for thin filmfilters having a negative temperature drift coefficient, in which passbandwidth broadens when temperature decreases and narrows whentemperature increases, as shown in FIG. 4. Note that in FIG. 4, the passbandwidth increases less at the most extreme temperatures than for theAlcatel 1915 LMI case, shown in FIG. 3.

[0007] Operational temperature fluctuation affects the stress present ina thin film filter, since film stacks and substrates of thin filmfilters are composed of different materials having differentcoefficients of thermal expansion (CTE). Thin film stack are depositedon substrates under temperatures substantially higher than roomtemperature, and then are allowed to cool down to room temperature. Ifthe CTE of a film stack is smaller than that of a substrate on which itis mounted, then the film stack will shrink less than the substrate doesas they cool down. Therefore, a convex deformation occurs and acompressive stress is induced in the film stack (see FIG. 1b). This isthe case of a stack-substrate combination having a negative temperaturedrift coefficient.

[0008] In nearly all prior art, DWDM thin film filters have positivetemperature drift coefficients. This is the situation illustrated inFIG. 1a. Because the thin film stack is deposited under a temperaturesubstantially higher than room temperature, when cooling down to roomtemperature, the film stack, which has a CTE greater than that of thesubstrate on which the film stack is mounted, shrinks more than thesubstrate does. Therefore, a concave deformation occurs. The film stackin this situation is under a tensile stress and pass bandwidth increasesas temperature increases, which causes greater susceptibility tocrosstalk as temperature increases. The tensile stress endured by thefilm stack is also a disadvantage during cutting operations, since itmakes the affected film layers more brittle, increasing the probabilityof damage to the film stack during cutting. Furthermore, the adhesionbetween the film stack and the substrate maybe overstressed, resultingin peeling of the film stack from the substrate.

SUMMARY OF THE PRESENT INVENTION

[0009] An object of the present invention is to provide thin filmfilters having a negative temperature drift coefficient and the methodmaking the same, thus promoting a narrowing of pass bandwidth astemperature rises within its operational temperature range.

[0010] Another object of the present invention is to provide a methodfor making thin film filters having film stacks which are under acompressive stress during cutting of the thin film filters.

[0011] Two embodiments of the present invented method for making thinfilm filters having a negative temperature drift coefficient aredisclosed. The first embodiment comprises steps of: 1. providing asubstrate wafer which has a coefficient of thermal expansion (CTE)greater than that of a selected film stack material; 2. polishing thesubstrate wafer; 3. depositing thin film layers made of the film stackmaterial on the substrate wafer at a temperature substantially higherthan room temperature; 4. cooling the substrate-film stack laminate toroom temperature, thus forming a convex-shaped laminate; 5. cutting thecooled laminate into pieces at room temperature. A second embodimentcomprises the steps of: 1. providing a laminate composed of a glasssubstrate and a film stack; 2. using at least one ion beam source tobombard the film stack of the laminate with high energy ions; 3. cuttingthe bombarded laminate into pieces.

[0012] Other objects, advantages and novel features of the inventionwill become more apparent from the following detailed description whentaken in conjunction with the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1a is a cross-sectional view of a thin film filter having apositive temperature drift coefficient of the prior art;

[0014]FIG. 1b is a cross-sectional view of a thin film filter having anegative temperature drift coefficient according to the invention;

[0015]FIG. 2 is a graph of a thin film filter's spectral transmittanceversus wavelength characteristics, showing a pass bandwidth of a thinfilm filter at room temperature (23° C.);

[0016]FIG. 3 is a graph of a thin film filter's spectral transmittanceversus wavelength characteristics, for the case of a thin film filterhaving a positive temperature drift coefficient, showing the change inpass bandwidth over the operational temperature range (−5° C. to 70°C.);

[0017]FIG. 4 is a graph of a thin film filter's spectral transmittanceversus wavelength characteristics, for the case of a thin film filterhaving a negative temperature drift coefficient, showing the change inpass bandwidth over the operational temperature range (−5° C. to 70°C.);

[0018]FIG. 5 is a schematic diagram showing the functioning of aneight-channel, filter-type DWDM device.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE PRESENT INVENTION

[0019] The present invention provides two embodiments of a method formaking thin film filters having a negative temperature driftcoefficient.

[0020] The first preferred embodiment of the present invented method formaking thin film filters having a negative temperature drift coefficientgenerally comprises five steps as follows: 1. providing a substratewafer which has a coefficient of thermal expansion (CTE) greater thanthat of selected film stack material; 2. polishing the substrate wafer;3. depositing a stack of films each having a CTE smaller than that ofthe substrate wafer onto the substrate at a temperature substantiallyhigher than room temperature; 4. cooling the resulting substrate-filmlaminate to room temperature, thus forming a convex-shapedsubstrate-film laminate; 5. cutting the cooled substrate-film laminateinto pieces.

[0021] In the first step, a substrate wafer that has a CTE ranging from10×10 ⁻⁶/° K to 20×10⁻⁶/° K is provided. And the substrate wafer must betransparent in the telecommunication range, i.e., C band (1528 nm to1561 nm) and L band (1561 nm to 1620 nm). The substrate wafer can bemade of glass of a SiO₂—Na₂O—K₂O—Li₂O—PbO—XO₂ system, wherein X can betitanium (Ti) or zirconium (Zr). It can also be made of aSiO₂—Na₂O—K₂O—Li₂O—PbO—Y₂O₃ system, wherein Y can be aluminum (Al), orof a SiO₂—Na₂O—K₂O—Li₂O—P₂O₅—ZO₂ system, wherein Z can be titanium (Ti)or zirconium (Zr). To increase the CTE of the glass substrate to thedesired range, the substrate wafer can be doped with lead (Pb), lithium(Li), sodium (Na), potassium (K), or some other alkali ions or oxides.

[0022] In order to increase the adhesion between the film stack and thesubstrate wafer, in the second step, we polish the substrate wafer to aroughness range of from 0.1 nm to 0.8 nm.

[0023] Then, in the third step, we use Ta₂O₅/SiO₂ which has a CTEranging from 1×10⁻⁶/° K to 8×10⁻⁶/° K, as a material for the thin filmstack deposited on the substrate. Each film layer is made of the filmstack material, and a chemical vapor deposition (CVD) method ispreferred for depositing the film layers on the substrate and on eachother. In this step, the substrate and film layers are substantiallyplanar during the layering process. And the process is conducted at atemperature substantially higher than room temperature.

[0024] In the fourth step, the substrate and film layers laminateproduced in step three is cooled down. Referring to FIG. 1b, since theCTE of the substrate wafer is greater than that of the film layers, whencooling down in the fourth step, the substrate wafer shrinks more thanthe film layers do. Therefore, the resulting laminated substrate andfilm layers will become slightly convex, and the film layers will endurea compressive stress at room temperature.

[0025] Finally, in the fifth step, the convexly shaped laminate of thesubstrate and film layers is cut into pieces, each having a negativetemperature drift coefficient and compressive stress distribution in itsfilm stack at room temperature.

[0026] The second preferred embodiment of the present invented methodfor making thin film filters having a negative temperature driftcoefficient comprises three steps as follows: 1. providing a laminatecomprising a glass substrate and a film stack; 2. providing at least oneion beam source for use in bombarding the film stack in the laminate; 3.cutting the bombarded laminate into pieces.

[0027] In the first step of the second embodiment, a laminate made of aglass substrate and thin film stack is put in a target position. In thesecond step, at least one ion source is heated to release ions. The ionsare then accelerated by an electric field to bombard the film stack inthe target laminate. Before reaching the target, the mean energy of theions is between 100 and 1500 electron-volts. The ion beam source may bea Kaufman source. The bombarding ion beam causes the structure of thefilm stack to condense. A denser structure means closer distancesbetween adjacent atoms in the film stack, which induces a compressivestress in the film stack. Finally, in the third step, the resultantbombarded laminate is cut into pieces to obtain the DWDM filter deviceswith a desired negative temperature drift coefficient.

[0028] The method described in the first preferred embodiment can beused with the second. Thin film filters made using the above describedmethods have a more dependable optical performance in a DWDM system whenworking within the operational temperature range. It is to be understoodthat the above-described preferred embodiments of the present inventionare intended to exemplify the invention without limiting its scope. Inaddition, even though numerous characteristics and advantages of thepresent invention have been set forth in the foregoing description,together with details of the functions of the present invention, thedisclosure is illustrative only. Changes may be made in detail,especially in matters of obviously similar methods, materials, processesand equipment, within the principles of the present invention to thefull extent indicated by the broad general meaning of the terms in whichthe appended claims are expressed.

1. A method for making a thin film filter having a negative temperaturedrift coefficient comprises the steps of: providing a film stackmaterial; providing a substrate wafer which has a coefficient of thermalexpansion greater than that of the film stack material; polishing thesubstrate wafer; depositing thin film layers made of the film stackmaterial on the substrate wafer at a temperature substantially higherthan room temperature, creating a film stack on the substrate wafer;cooling the substrate wafer-film stack laminate to room temperature;cutting the cooled substrate wafer-film stack laminate into pieces. 2.The method as described in claim 1, wherein the coefficient of thermalexpansion of the substrate is within the range from 10×10⁻⁶/° K to20×10⁻⁶/° K.
 3. The method as described in claim 1, wherein thesubstrate wafer is transparent to electromagnetic waves having awavelength in a range between 1561 nm and 1620 nm.
 4. The method asdescribed in claim 2, wherein the substrate is made of aSiO₂—Na₂O—K₂O—Li₂O—PbO—XO₂ system, wherein X can be titanium (Ti) orzirconium (Zr).
 5. The method as described in claim 2, wherein thesubstrate is made of a SiO₂—Na₂O—K₂O—Li₂O—PbO—Y₂O₃ system wherein Y canbe aluminum (Al).
 6. The method as described in claim 2, wherein thesubstrate is made of a SiO₂—Na₂O—K₂O—Li₂O—P₂O₅—ZO₂ system, wherein Z canbe titanium (Ti) or zirconium (Zr).
 7. The method as described in claim2, wherein the substrate wafer is doped with at least one of a groupcomprising lead (Pb), lithium (Li), sodium (Na) and potassium (K). 8.The method as described in claim 1, wherein the surface of the substratehas an average roughness range of between 0.1 nm and 0.8 nm.
 9. Themethod as described in claim 1, wherein the materials making the filmstack are tantalum oxide and silicate dioxide.
 10. The method asdescribed in claim 1, wherein the film stack endures a substantiallycompressive stress at room temperature.
 11. A method for making a thinfilm filter having a negative temperature drift coefficient comprisesthe steps of: providing a laminate comprising a glass substrate and afilm stack formed thereon; using at least one ion beam source to bombardthe film stack of the laminate when the laminate is in a targetposition, wherein the at least one ion beam source is heated to releaseions and the ions are accelerated by an electrical field; cutting thebombarded laminate into pieces.
 12. The method as described in claim 11,wherein the at least one ion beam source is a Kaufman source.
 13. Themethod as described in claim 11, wherein, before reaching the targetlaminate the mean energy of the accelerated ions is within the rangefrom 100 to 1500 electron-volts.
 14. The method as described in claim11, wherein, after being bombarded, the substrate endures asubstantially tensile stress at room temperature.
 15. The method asdescribed in claim 11, wherein, after being bombarded, the film stackendures a substantially compressive stress at room temperature.
 16. Astructure of a film filter comprising a plurality of film layers made ofa film stack material and a substrate wafer which said film layers aredeposited on at a temperature substantially higher than a roomtemperature and is retained to after being cooled to the roomtemperature, wherein said substrate wafer owns a coefficient of thermalexpansion greater than that of the film stack material so as to generatecompressive forces upon the associated film layers, thus resulting in anegative temperature drift coefficient of said film filter.
 17. Astructure of a film filter comprising a plurality of film layers made ofa film stack material and a substrate wafer which said film layers aredeposited on and retained to; wherein said substrate wafer owns acoefficient of thermal expansion greater than that of the film stackmaterial, and from a microscopic viewpoint both said substrate wafer andsaid film layers commonly define a convex configuration under acondition that said substrate wafer is located closer to a center ofsaid convex configuration than said filter layers.
 18. A structure of afilm filter comprising a plurality of film layers made of a film stackmaterial and a substrate wafer which said film layers are deposited onand retained to, said substrate wafer owning a coefficient of thermalexpansion greater than that of the film stack material and resulting ina negative temperature drift coefficient of the film filter incomparison with a positive temperature drift coefficient of aconventional film filter, said film filter being characterized in that:within a range between a higher temperature and a lower temperature withtherebetween a room temperature corresponding to a normal bandwidth,said film filter at said lower temperature results in thereof a narrowedbandwidth with a reduced value smaller than another reduced value ofanother narrowed bandwidth resulting from said conventional film filterat said higher temperature, and said film filter at said highertemperature results in thereof a broadened bandwidth with thereof anincreased value smaller than another increased value of anotherbroadened bandwidth resulting from said conventional film filter at saidlower temperature.